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Official diet advice on fat stands, despite new study

Claims that dietary fat guidelines in the US and UK were not based on good scientific evidence are misguided, says Christine Williams

Yet again questions are being asked about the long-standing advice to avoid diets rich in fat, especially saturated fats.

The latest doubts have been prompted by a study that says randomised controlled trials (RCTs) available when dietary guidance on fat was introduced – in 1977 in the US and 1983 in the UK – did not support that guidance (Open Heart, DOI&colon; 10.1136/openhrt-2014-000196).

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Both nations urged people to eat less fat, especially the saturated kind, pointing to its potential to raise a type of cholesterol in blood that can lead to coronary heart disease. This official advice is still in place. Rightly so.

The problem is, the authors of the latest study have taken a classical pharmaceutical approach, with the assumption that RCTs provide the gold standard for dietary studies as they do in clinical drug trials. They don’t. In fact such an approach would be inappropriate for most population-based recommendations.

Part of the picture

Most dietary guidelines have been developed using the degree of consistency of a number of lines of evidence as the best way to assess risk. RCTs are part of that, but only a part.

The problem is that drug trials usually involve patient groups likely to benefit from the tested drug. In contrast, dietary guidelines aim to prevent disease in a healthy population.

The results from dietary RCTs involving people with disease may not apply to healthy people. And to do RCTs on healthy people requires many more volunteers studied over very long periods – usually many years. Sticking to strict diets is challenging, especially when they involve changes in many common foods and drop-out rates can be very high. So cost, commitment of volunteers, feasibility and uncertainty about the reliability of the findings are all major challenges.

Despite this, when the dietary fat guidelines were introduced in the UK and US, consistent evidence for the cholesterol raising effects of saturated fats was available from animal studies, comparisons of health outcomes between countries with differing diets, and long-term observational studies of large numbers of people.

The drugs do work

In around 1970, cell studies were beginning to show the mechanisms by which dietary fats affect cholesterol levels in blood. Over the next decade, this fundamental biology formed the basis of both dietary advice and the development of statin drugs that lower cholesterol to reduce heart disease risk. It’s hard to deduce that statins work but the diet does not.

In addition, strong evidence was available from trials in metabolic wards, where volunteers on strict short-term diets could be studied, and from volunteers in semi-free-living settings, where there was some degree of dietary control. These showed that the effects of saturated fats on cholesterol were so consistent that the investigators were able to develop a predictive algorithm to estimate the cholesterol outcome based on the saturated fat in the diet.

The RCT studies discussed in the Open Heart paper were available to the expert committees at the time the guidelines were drawn up. They weren’t ignored. Their findings were, in part, responsible for the decisions not to recommend complete replacement of dietary saturated fats with polyunsaturated fats, because of possible adverse effects of higher intakes of these fats.

While it is important that we continue to look at all the evidence on diet and heart disease, this must be through a holistic assessment and recognition of the clear improvements in population cholesterol levels over the past 30 years.

The dietary guidance is working in that sense. Of course there are justifiable concerns that saturated fats have been replaced in the diet by sugars and this requires a reasoned assessment of alternative approaches for the future.

But any suggestion that eating more fat than advised is OK remains wishful thinking – and could be harmful for heart health.